The present inventors had made a radial piston pump as shown in FIG. 3, the radial piston pump has a rotor 2 rotatably provided on a pintle portion 1, and the pintle portion 1 is provided within a cylinder portion 30 formed within a housing 3. A bearing 4 is provided between the inner surface of the housing and the outer surface of the rotor 2. A piston 6 is slidably provided within a working chamber formed in the rotor 2, so that the piston 6 is rotated simultaneously with the inner race of the bearing 4. Since the center portion of the rotor 2 is offset from the center portion of the cylinder portion 30, the piston 6 is reciprocated within the working chamber 8 while the rotor 2 is rotated, so that the oil is introduced into the working chamber from a suction groove 9 and discharged from the working chamber 8 to a discharge groove 10 in accordance with the reciprocation of the piston.
It is eagerly required to reduce the noise generated by the radial piston pump, especially when the pump is mounted on an automobile. In order to reduce the noise generated by the pump, the present inventors had studied about the relationship between the discharge pressure and the noise sound pressure of the pump. As shown from FIG. 5, the sound pressure has peaks at times that coincide with the peaks of the discharge oil pressure. Accordingly, the peak of the sound pressure occurs when the discharge oil pressure reaches its maximum.
It is the present inventor's opinion that the highly pressurized oil within the discharge groove 10 flushes back toward the working chamber 8 when the front edge of the working chamber 8 contacts the discharge groove 10 as shown in FIG. 6, and the back flow of the oil makes the noise sound pressure.
In order to cease the back flow, the present inventors had made the pump the front edge 11 of that had a discharge groove 10 of which is withdrawn as shown in FIG. 7. Namely, the pump shown in FIG. 7 has a delay angle .THETA.c between a bottom dead point X at which the protruding amount of the piston from the rotor 2 becomes maximum and a connecting point Y at which the front edge 12 of the working chamber 8 contacts with the edge 11 of the discharge groove 10. Since the oil within the working chamber 8 is preliminary pressurized during the delay angle .THETA.c, the oil pressure within the working chamber 8 becomes as high as that within the discharge groove, to present the oil within the discharge groove flushing back to the working chamber 8. FIG. 8 shows the pattern of the discharge pressure of the pump shown in FIG. 7. As clearly shown from FIGS. 5 and 8, the vibration of the discharge pressure can be reduced when the pump has the delay angle .THETA.c.
The noise sound pressure of the pump having the delay angle, however, can not be reduced as much as the discharge pressure, so that the total noise generated by the pump having the structure such as shown in FIG. 7 can not reduce the generation of the noise effectively. The present inventors, therefore, have further studied the mechanism of the noise production. As shown in FIG. 4, the piston 6 and the shoe 7 of radial piston pump rotate simultaneously with the inner race of the bearing 4 when the rotor 2 is rotated, in other words, the relative movement between the piston 6 and shoe 7 and also between the shoe 7 and the inner race of the bearing does not occur even while the rotor 2 is rotated. Accordingly, it is very hard for the oil to be introduced between the shoe 7 and the inner race of the bearing.
If the bearing 7 slides along with the inner surface of the bearing 4, the oil may be introduced between the inner race of the bearing 4 and the shoe 7, so that the oil held between the inner race and the shoe can interrupt the transmittal of the vibration from the piston 6 to the housing 3.
On the other hand, since the shoe 7 of the radial piston pump rotates simultaneously with the inner race of the bearing as described before, no oil can exist between the shoe 7 and the bearing 4. Therefore, the vibration of the piston can be transmitted to the bearing 4 through the shoe 7, so that the vibration of the piston 6 is directly transmitted to the housing 3.
The present inventors have determined that the vibration of the piston 6 is the primary noise source of the radial piston pump having the bearing 4 between the piston 6 and the housing 3. The present inventors had studied about the relationship between the vibration of the housing 3 and the rotation of the rotor 2. In order to detect the vibration of the housing 3, an acceleration sensor 33 is attached on the housing as shown in FIG. 9. FIG. 10 shows the acceleration signal detected by the acceleration sensor 33. As shown from FIG. 10, the maximum point of the vibration of the housing 3 is not occurred at the bottom dead point of the piston 6 but occurred at the top dead point of the piston 6, the bottom dead point of the piston 6 is identical with the position X shown in FIG. 7, and the top dead point of the piston 6 is identical with the position that the retreating amount of the piston 6 into the working chamber 8 becomes maximum.
The present inventors had then studied about the vibration source of the housing 3. Since the radial piston pump shown in FIG. 4 has three pistons and since there are only two pressure conditions e.g. the discharge pressure condition and the suction pressure condition while the rotor rotates, there should be exist two types of pressure balances. One is that two of three pistons 6 receive the discharge pressure and the remaining piston receives the suction pressure. Another is that one of three pistons receives the discharge pressure and remaining two pistons receive the suction pressure. FIG. 11 shows the former condition where the pistons 61 and 62 receive the discharge pressure so that the resultant of the two forces are occurred at the point K which locates between the pistons 61 and 62. FIG. 12 shows the latter condition that only one piston 62 receives the discharge pressure, so that the force point K is identical with the position of the piston 62. Since the condition shown in FIG. 12 occurs immediately after the rotor 2 rotates from the condition shown in FIG. 11, the force point K shifts quickly and this makes the housing 3 vibrate. It is, therefore, the present inventors opinion that the vibration of the housing 3 should be reduced if the force point K where shift slowly from the point shown in FIG. 11 to the point shown in FIG. 12. In order to attain the slow shift of the force point K, the present inventors have provided a radial piston pump having a front edge of the working chamber 8 which does not contact with the suction groove 9 at the first point W which is identical with the top dead portion of the piston 61 but is contacted with the suction groove at the second position Z as shown in FIG. 13. Accordingly, the oil pressure within the working chamber 8 is preliminary reduced during a delay angle .THETA.e between the first position W and the second position Z.
In order to determine the amount of the delay angle .THETA.e, the present inventors calculated the angle .THETA.e by using the following formula (1). EQU H(.THETA.e)=.epsilon.(1-Cos .THETA.e)+(R-r) (1-Cos .UPSILON.)--(1)
The varying rate P1 of the pressure within the working chamber 8 is calculated by using the formula (2). ##EQU1##
The introducing amount of the oil into the working chamber is calculated by using the formula (3). ##EQU2##
The varying rate of the pressure of oil the within the working chamber during the delay angle .THETA.e is calculated by using the formula (4). ##EQU3##
The amount of the oil introducing into the working chamber 8 is calculated by using the formula (5). ##EQU4##
In these formulas (1)-(5), .epsilon. represents the offset amount, .UPSILON. represents the rotating angle, R represents the inner diameter of the bearing, E represents the elastic coefficient of the oil, r represents the radius at the top portion of the piston, c represents flow coefficient, .rho. represents the density, .zeta. represents the coefficient of the pressure drove.
FIG. 14 shows the relationship between the delay angle .THETA.e and the pressure P.sub.1 within the chamber, the relationship being obtained by using the above mentioned formulas (1)-(5). As shown from FIG. 14, the pressure within the chamber 8 is gradually decreased when the delay angle .THETA.e becomes 41.degree., and the pressure P1 within the chamber 8 drops sharply when the delay angle .THETA.e becomes 10.degree.. FIG. 15 shows the relationship between the rotating angle .THETA. and the pressure varying rate within the working chamber 8. As shown from FIG. 15, the varying rate of the pressure within the chamber 8 can be small when the delay angle .THETA.e becomes 41.degree.. As described above, the the varying rate of pressure within the chamber is strongly dependant on the delay angle .THETA.e. FIG. 16. shows the influence of the delay angle .THETA.e. The ordinate of FIG. 16 is the delay angle .THETA.e and coordinate of FIG. 16 is the maximum varying rate of the pressure when the delay angle .THETA.e is fixed as the value indicated by the ordinate. As shown from FIG. 16, the varying rate of pressure becomes minimize when the delay angle .THETA.e becomes 40.degree..
FIG. 17 shows the movement of the forcing point K at which the pressing force caused by the pistons is focused when the delay angle .THETA.e is 41.degree.. As shown from FIG. 17, the forcing point K is varied in accordance with the rotation of the rotor 2, and the movement of the forcing point K is very gradually. FIG. 18 shows the vibration of the housing of the radial piston pump the delay angle .THETA.e of which is 41.degree.. It is understood that radial piston pump having the delay angle .THETA.e of 41.degree. can reduce the vibration thereof. FIG. 19 shows the level of the radial piston pump which has the same features as that shown in FIGS. 10 and 18. The solid line A indicates the noise level of the radial piston pump having the delay angle .THETA.e of 41.degree., the dotted line B indicates the noise level of the radial piston pump having no delay angle .THETA.e. The noise level of the radial piston pump having the delay angle .THETA.e of 41.degree. can be reduced.
It should be also noticed that the noise level of the pump when the discharge pressure 21 MPa is higher than that when the discharge pressure is 14 MPa. Since the delay angle .THETA.e of 41.degree. is obtained from the calculation by using the formulas (1)-(5), for the purpose of reducing the noise level when the discharge pressure is 21 MPa, the actual data obtained from the test pieces is slightly different from that calculated value by the formulas. It is the present inventors opinion that the difference between the actual data and the calculated value is caused by the oil leakage that occurs around the piston. In order to reduce the noise level of the pump, the present inventors made several test pieces the delay angle .THETA.e of which are different from each other. The solid line C of FIG. 20 indicates the noise level of the pump the delay angle of which is 43.degree., the solid line D of FIG. 20 also indicates the noise level of the pump the delay angle .THETA.e of which is 53.degree., the solid line E of FIG. 20 indicates the noise level of the pump the delay angle .THETA.e of which is 63.degree.. As shown from FIG. 20, the noise level of the pumps having the delay angle larger than 43.degree. are not different from each other. The minimum point of the noise level, on the other hand, is valid in accordance with the delay angle .THETA.e.
The present inventors had then studied about the relationship between the delay angle .THETA.e and the discharge pressure at which the noise level becomes minimize. The discharge pressure making the noise level minimize is strongly depend on the increment of the delay angle .THETA.e as shown in FIG. 21. Since the operable range of the pump is the range between 14 MPa and 21 MPa, the delay angle .THETA.e should be in the range of 43.degree.-63.degree.. Since the pump efficiency should be decreased when the delay angle .THETA.e becomes too large, the delay angle .THETA.e of less than 60.degree. is preferable.
The present inventors have completed the present invention after studying about the noise generated by the pump. The present invention, therefore, has the object to reduce the noise level of the pump.
In order to attain the object, the radial piston pump of the present invention employs such features that the front edge of the working chamber begins to contact with the suction groove after the rotor is rotated by the predetermined delay angle .THETA.e from a first position at which the piston locate the top dead point thereof. The delay angle .THETA.e during which the pressure within the working chamber is reduced is practical when the angle is more than 40.degree.. It is well prevented that the forcing point caused by the piston is moved quickly, so that the vibration of the housing is also well prevented.